This invention relates to the field of medical imaging, and particularly to a technique for variably altering the appearance of compensated radiological images, such compensated images being acquired by a device that equalizes the average exposure through different portions of the patient. The present invention allows the diagnostician to optimize an x-ray image by tuning, retrospectively, the amount of dispayed equalization between different portions of the image.
While the present invention will be described in terms of x-ray imaging with respect to chest x-rays, it is to be understood that the teachings of this invention are applicable to a wide range of medical imaging fields.
The field of projection x-ray imaging is well developed and such x-ray equipment is now installed in all hospitals throughout the United States. A problem with known projection x-ray imaging techniques is that the x-ray image transferred to a film recording medium tends to have a great variation in exposure across the image. For example, referring to FIG. 1, a chest x-ray (radiograph) is seen to present widely contrasting light and dark areas. This exposure problem is much like that presented in 35 mm photography, and other imaging systems where the dynamic range of the detecting media (film, etc.) is not as wide as the dynamic range of information available in the imaged object.
In the radiological field, there is often a very wide range of x-ray intensity transmitted through different parts of the patient, particularly radiographic exams such as chest radiographs. This wide variation in transmitted intensity, known as wide dynamic range, leads to degradation of the final image. This degradation occurs because of wide variations of signal-to-noise ratio (SNR) across the image, because of limited film latitude, and because the eye-brain system has a difficult time accommodating wide dynamic ranges.
In recent years, much attention has been given to the optimization of chest radiographs. The chest radiograph is a powerful tool for assessing diseases of the thorax, but it is subject to various physical and perceptual limitations which degrade its diagnostic potential. Two of the main contributors to reduced diagnostic accuracy are large image dynamic range and low lesion conspicuity. Conspicuity is the conspicuousness with which a particular structure appears in the image, compared to the background structure. The large image dynamic range results from the wide disparity in x-ray attenuation between the various structures present in the patient. For example, referring to FIG. 1 it is seen that the lung region is very transmissive to x-rays, while the dense mediastinum areas greatly attenuate x-rays. This is because the mediastinum contains a plurality of structures such as the backbone, aorta, heart, esophagus, bronchial tubes, etc., the whole of which contributes to greater x-ray attenuation than the air-filled lungs. Thus, the chest presents a wide range of x-ray attenuation variations between the lung field and dense regions such as the mediastinum and sub-diaphragmatic areas. The wide range of transmitted and detected x-ray intensity places physical constraints on the imaging system, and also aggravates the psycho-visual process of lesion detection. Poor conspicuity, on the other hand, is the result of the complex network of superposed anatomical structures against which the lesion must be visually discerned. For example, in chest radiographs it is often difficult to discern the presence of small tumors against the mottled background of normal lung structures.
There are three main problems associated with excessive image dynamic range. First, there is a compromised SNR in poorly penetrated regions; it is not uncommon for there to be a 7-fold variation in SNR over the area of a chest radiograph. The problem of reduced SNR in poorly penetrated regions is compounded by the increased contribution of detector noise with large dynamic range. A second problem with large dynamic range is the inability of certain detectors to record the full range of incident intensity. Typical film/screen combinations, for example, do not have sufficient latitude to record the full dynamic range of the chest, resulting in reduced contrast in the poorly penetrated regions. Thirdly, large dynamic range presents display problems since it is difficult to adequately visualize a wide range of image brightnesses simultaneously.
Various approaches have been investigated for eliminating the problems associated with large dynamic range. Generalized procedures such as increasing the kilovoltage of the x-ray source, or using wide-latitude film have been tried, but it is not clear that diagnostic improvement will be achieved since these techniques reduce the contrast of all structures. In addition, digital filtering procedures are also being investigated, but they are limited in their enhancement by the degraded SNR properties of poorly penetrated regions. Various mechanical and photographic techniques have also been tried and have generally yielded improved results. One such technique is a photographic unsharp-masking technique which improves mediastinal nodule detection two-fold. However, this technique also requires fairly extensive film handling and twice the patient exposure. Also, a variety of automated beam filters have been proposed, but many are fairly mechanically cumbersome. Some of these mechanical techniques are generally described in U.S. Pat. No. 3,755,672, to Edholm et al., the teachings of which are also incorporated herein by reference.
Recently, several promising mechanical techniques have been formulated for reducing the dynamic in chest radiography. Notable examples are the Digital Beam Attenuator (DBA), Scanning Equalization Radiography (SER), and the Oldelft Compensator, sometimes known as AMBER (Advanced Multiple Beam Equalization Radiography).
Each of the DBA, SER, and AMBER devices compensates the x-ray beam for perceived x-ray attenuation variations within the object. Briefly, each of these techniques determines the x-ray transmissivity of every point in an object, and then spatially modulates the x-ray beam in accordance with the detected attenuation variations within the object. For example, the DBA technique first takes a low-dose x-ray image of the object. Then, an attenuation filter, or mask, is constructed from the low-dose image in order to reduce the x-ray flux incident on the lung region more than the x-ray flux incident on the mediastinum. For example, the DBA mask (template) may comprise a sheet of paper on which radiation-attenuating materials, such as Cerium Oxide, are imprinted. The DBA mask is then positioned between the object and the x-ray tube, and a second, compensated, image of the object is taken. The compensated image at the detector now has an exposure which is much more uniform across the object, for example see FIG. 3. The DBA technique is more fully describe in U.S. Pat. No. 4,497,062, to Mistretta, et al. (of which the present inventor was a co-inventor), the teachings of which are incorporated herein by reference. In addition, the article "Digital Beam Attenuator Technique for Compensated Chest Radiography", by Hasegawa et al., appearing in Radiology, Vol. 159, Number 2 pp 537-543, 1986, also fully describes the DBA technique, and is also incorporated herein by reference.
The DBA technique is successful at reducing the overall dynamic range in chest radiography, resulting in improved uniformity of image SNR, and improved visualization of mediastinal structures. The DBA technique utilizes 2-dimensional (area) modulation of the incident x-ray beam. This technique provides good temporal resolution since short exposure time (50-100 ms) may be used, during which time patient structures are relatively still. Furthermore, this technique is an efficient use of the x-ray source and reduces x-ray tube loading, resulting in longer tube life and improved reliability. However, two potential difficulties exist with the DBA technique. First, the finite length of time (approximately 50 seconds) required to generate the beam attenuator mask and then place it in the x-ray beam to make the compensated image leads to potential misregistration artifacts between the attenuator mask and the patient. Thus, if the patient moves between generation of the attenuator mask and formation of the compensated image, an inaccurate image will result. This is demonstrated in FIG. 4 where an intentional misregistration between the attenuator and patient was performed to observe the effect. Notice the extreme light and dark areas in the right portion of FIG. 4. Secondly, the DBA technique is mechanically complex, requiring a mechanical printer to print the attenuator mask and a radiation attenuating (for example Cerium Oxide) ribbon for use with the printer. Cerium Oxide ribbons have exhibited a wide range of reliability, thus leading to potential problems with attenuator mask generation.
The SER technique is also a beam compensation technique capable of producing more uniform exposure in a radiological image. In the SER technique, radiation-blocking plates are disposed between the x-ray tube and the patient. One plate has a vertical slot, while the second plate has a horizontal slot, thus providing a pencil-shaped beam of radiation through the overlapping plates. The plates are X-Y driven to produce a x-ray beam raster scan pattern of the pencil beam across the object to be imaged. Behind the object, a feedback detector is located which detects the intensity of x-rays transiting the object. When low x-ray intensity is detected, this indicates a great deal of x-ray attenuation within the object. The feedback detector provides a signal to an x-ray tube modulator which increases the net x-ray flux to compensate for the attenuation within the object. The SER technique is thus a 0.sup.th dimensional (point) method of beam modulation. The SER modulates the x-ray tube output by changing the length of x-ray pulses, thus requiring a more expensive and complicated x-ray generator and control mechanism. In addition, the SER technique suffers from mechanical complexity from the driving mechanism for the plates in the X-Y direction. Finally, the SER technique demands a great deal of x-ray tube output since only a small portion of the generated x-ray beam actually reaches the object. With replacement x-ray tubes costing approximately $15,000, this technique presents economic disadvantages. The SER technique is more fully described in the articles: "A Scanning Equalization System for Improved Chest Radiology", by Plewes et al. in Radiology, Vol. 142, pp 765-768, 1982; "A Scanning System for Chest Radiography with Regional Exposure Control: Theoretical Considerations", by Plewes in Med. Physics, Vol. 10, pp 646- 654, 1983; and "A Scanning System for Chest Radiography with Regional Exposure Control: Practical Implementation", by Plewes et al., in Med Physics, Vol. 10, pp 655-663, 1983, the teachings of which articles are incorporated herein by reference.
The AMBER technique of radiographic compensation is somewhat like the SER technique in that a feedback detector detects x-ray attenuation within the object and then alters the x-ray flux on the object to compensate for x-ray attenuation within the object. The AMBER technique utilizes a plate with a single horizontal slot containing a plurality of vertical occluders therein to attenuate the x-ray beam. The plate produces a plurality of pencil beams which are scanned vertically over the object. Each occluder within the plate includes means for modulating the amount of x-rays passing through that particular position of the slot. There is a feedback detector for each of the pencil beams transiting the object. Each of the feedback detectors is coupled to a respective one of the slot modulating means. As each detector determines the amount of x-ray transmission through the object, it modifies its respective occluder width in order to increase or decrease the amount of x-rays incident on the object. As the x-ray beam scans the object, modulation of the x-ray pencil beams produces various amounts of x-ray intensity in the resultant image. Thus, the AMBER technique is a one-dimensional (line) system of x-ray beam modulation. The AMBER device is believed to have been reduced to practice commercially and is capable of producing usable compensated x-ray images. However, the AMBER technique produces vertical streaking artifacts in the image, between the vertically scanned strips. Moreover, the AMBER technique also makes inefficient use of the x-ray tube since a good portion of the x-ray tube output is blocked by the plate or the slot modulating devices. Again, x-ray tube loading is high. Finally, the AMBER technique is also complicated electrically, requiring the uniformity of response for the plurality of channels (for example, 19 channels) to be carefully coordinated. A more detailed description of the AMBER technique was provided at the Chest Imaging Conference-87 in presentations entitled "Multiple Beam SER System-Technical Aspects of AMBER", by Boelens et al., on Aug. 31, 1987; and "Multiple Beam SER System-Clinical Results", by Ravin on Aug. 31, 1987. Abstracts of both these presentations exist in the Chest Imaging Conference Program, and articles on the presentations are now being prepared, both the articles and their abstracts being also incorporated herein by reference.
In addition, it is expected that future beam compensation techniques will be devised so that exposure equalized radiological images may be provided to diagnosticians. However, all beam compensation devices, present and future, share certain disadvantages. First, in the compensation process eliminates certain large-area information from the image, which information may be diagnostically important. For example, general lung opacity is reduced or eliminated in the compensated image, resulting in potentially degraded diagnosis of large area lung disease. FIG. 3, trace 10 depicts the image signal value across the horizontal midline of the chest. This trace demonstrates that all large area information has been removed from the image, leaving only the small, detailed structures. Secondly, when extreme compensation is carried out considerable edge artifacts (black and white bands) at the heart/lung and diaphragm/lung boundaries may appear. For example, in FIG. 3 it can be seen that certain dark bands exist around the large mediastinal organs. These edge artifacts may prevent accurate diagnosis of phenomena existing in the boundary regions. The reason for such edge artifacts is the high-pass filtration of the x-ray beam from the beam compensation, thus reducing large structure signal while leaving small structure signal intact. The dips in the signal profile in FIG. 3, trace 10 at the edge of the heart are indicative of the edge artifacts from compensation. Thirdly, the beam compensation technique itself may introduce artifacts, such as the vertical streak artifacts produced by AMBER, or the misregistration artifacts produced by the DBA technique. In addition, aggressive compensation in any beam compensation technique may distort the lung images and make a visually "busy" image, resulting in a conspicuity problem in which the background information is confusing and psycho-visual degradation may interfere with the diagnosis. For example, in FIG. 3 the lung areas 16 have been aggressively compensated to make their exposure nearly equal to that of the mediastinum area 18. This may present a busy image and actually interfere with the diagnosis of phenomena in any of the areas. Finally, none of the known or future beam compensation techniques allows for the retrospective alteration of the degree of compensation to fit the specific problem which is the subject of diagnosis. Without the ability to change the degree of compensation, the diagnostician must choose a compromise between improved mediastinal appearance from exposure-equalization, and the degrading effects of edge artifacts and compensator artifacts. Thus, what is needed is a technique which allows the diagnostician to retrospectively alter the degree of displayed compensation in any image in order to choose the exact image best suited to the particular diagnosis being considered.
Thus, what is needed is a reliable, inexpensive system adaptable for use with any beam compensation technique which allows the diagnostician to retrospectively alter the degree of displayed image compensation while maintaining the other, beneficial aspects of compensation techniques.